1. Field of the Invention
This invention relates to cationic group-containing vinyl alcohol copolymers, methods of producing the same and paper strength additives comprising the same.
2. Description of the Prior Art
Polyvinyl alcohol (hereinafter PVA) is a typical water-soluble synthetic polymer and has been used widely for various industrial purposes, particularly as a raw material in the production of a synthetic fiber VINYLON and as a size. The film-forming properties and strength characteristics of PVA are especially taken advantage of in paper surface sizing and warp sizing. PVA is also expected to be effective as a paper strength additive, that is a size to be used internally in papermaking for the purpose of increasing the strength of paper. However, PVA has not yet been applied as such an agent. This is because PVA is one of the so-called nonionic polymers which essentially do not contain any ionic groups and therefore PVA, in substance, cannot be adsorbed by the pulp even if an aqueous solution of PVA is added to the pulp slurry. According to Japanese Patent Publication Nos. 401/1971 and 38601/1971, PVA has been modified by the introduction of anionic groups, such as carboxyl, and the modified PVA's are used as strength additives for paper by causing them to be adsorbed by the pulp with combined use of alum. Such a modified PVA has never been commercialized as an internal strength additive for paper, however, presumably because the anionic groups in the modified PVA from intramolecular ester bonds with the hydroxyl groups under acidic conditions created by the combined use of alum. Since the anionic groups cannot serve as effective active sites, the intended improvement in fixation of the additive by the pulp cannot be effected. Cationic groups, instead of anionic groups, are also capable of being fixed by the pulp and it is known that under such conditions it is unnecessary to use alum. Cationic group-modified starch and polyacrylamide have already been commercialized. One might expect by analogy that when cationic groups are introduced into PVA, adsorption by the pulp would become possible and the excellent strength characteristics of PVA could be utilized effectively. However, as will be discussed, the known methods of introducing cationic groups into PVA have many difficulties and at present there are no measures available which can be adopted on a commercial scale.
In one known method of obtaining a cationic PVA, a cationic monomer, vinylpyridine and vinyl acetate were copolymerized and then subjected to saponification to give a modified PVA containing 1.19 mole % of vinylpyridine units (Kobunshi Kagaku 8, 467 (1951)). As described in this report, the rate of polymerization in this copolymerization system is so slow that the production of the copolymer is far from economical. In Kogyo Kagaku Zasshi 59, 658 (1956) and ibid. 60, 353 and 1188 (1957), methods are disclosed comprising copolymerizing vinyl acetate and N-vinylphthalimide or N-vinylsuccinimide, then saponifying the vinyl acetate units and further decomposing the imide groups with an alkali or hydrazine. The imide groups of both the imide monomer units can be decomposed to amide groups but, as shown in the above-cited reports, it is difficult to make the degradation reaction proceed until cationic amino groups are formed.
Various methods of making PVA cationic by methods other than copolymerization have also been proposed. For instance, it is reported that an amino group-containing PVA can be synthesized through aminoacetalization, e.g., Japanese Patent Publication Nos. 5563/1955, 3319/1956 and 23875/1978, Kobunshi Tenbo 15, 69 (1951), Kobunshi Ronbun Shu 34, 843 (1977), Journal of Applied Polymer Science 21, 2125 (1977) or through aminobenzacetalization, e.g., Japanese Patent Application Laid Open No. 38383/1976. This kind of PVA shows cationic properties only under limited conditions, namely in acidic aqueous solutions. In U.S. Pat. No. 3,345,346, a method of producing an amino group-containing PVA by an ether exchange reaction is disclosed, which comprises reacting PVA with an alkoxydimethylamine in anhydrous dioxane. Another method comprises subjecting PVA to a Michael addition reaction of acrylamide and then subjecting the acrylamide units to Hoffman degradation to introduce amino groups (Bulletin of Chemical Society of Japan 47, 2990 (1974)). A report (Nippon Kagaku Kaishi 1975 (11), 1955) describes a modified PVA with a nitrogen content of 2.6 to 5.1%, which was synthesized by treating PVA with sulfuric acid, reacting the so-treated PVA with epichlorohydrin and further with a polyethylenepolyamine. The structure of the product is not fully clear, but the product is claimed to be effective as a coagulant. Japanese Patent Laid Open No. 3689/1977 discloses a method of producing a quaternary ammonium salt-containing cationic PVA which comprises reacting glycidyltrimethylammonium chloride with PVA in the presence of an alkali catalyst. A similar process is, in fact, in commercial use to make starch cationic, but PVA, although it is also a polymer having hydroxyl groups, is much less reactive than starch to glycidyltrimethylammonium chloride or to a ring opened product thereof, namely, 3-chloro-2-hydroxypropyltrimethylammonium chloride. Such a process has not yet been commercially utilized with PVA.
Of the above prior art techniques, the polymer modification methods of introducing cationic groups, although realizable in laboratories, have the following difficulties from the industrial point of view:
1. It is necessary to dissolve PVA prior to the reaction. It is often difficult to attain homogenous reactions when PVA is subjected to the reaction in a powder or slurry form.
2. It is difficult to obtain products with a certain constant degree of modification.
3. Crosslinking reactions frequently occur simultaneously, leading to insolubilization of the polymers.
4. Generally, the operational costs are high.
On the other hand, none of the methods involving copolymerization, as mentioned above, have been found effective in providing PVA with cationic properties.
Vinylbenzyltrimethylammonium chloride is one commercially available cationic monomer. This monomer, however, is substantially uncopolymerizable with vinyl esters. Other commercially available cationic monomers are aminoalkyl (meth)acrylates such as: ##STR3## and are in wide use in producing cationic copolymers by copolymerization with a variety of monomers. These aminoalkyl (meth)acrylates are copolymerizable with vinyl esters, especially vinyl acetate. However, when the copolymer is saponified, the ester linkage in the aminoalkyl (meth)acrylate unit is also saponified simultaneously and therefore, it is difficult to effectively prepare the PVA containing cationic groups.
Heretofore, it has been difficult to produce modified polyvinyl alcohol having cationic properties on an industrial scale in an inexpensive manner. The prior art contains no effective means for doing so. A need, therefore, continues to exist for novel water soluble cationic copolymers containing PVA and for methods of preparing them.